The invention relates to an optoelectronic sensor, in particular a reflection light scanner, at least with one transmitter for transmitting transmission light, with a receiver for outputting at least two received signals in dependence on reflected and received transmission light, and with an evaluation device by which a difference of the received signals to a threshold can be compared to determine whether an object or no object is disposed in a detection zone of the sensor, and a positive or a negative article detection signal can be produced in dependence on the result of this comparison, with a plurality of different thresholds being provided to produce a switching hysteresis. The invention further relates to a corresponding method.
Sensors of this kind are intended to detect whether and when an object enters into the detection zone and/or exits from this, whereas objects in other spatial zones should be ignored. For example, in the case of so-called background masking, the detection zone can be a foreground zone; if an object moves from the foreground zone into the background zone, the sensor should switch from a positive to a negative article detection signal.
Sensors of this kind usually work in accordance with the triangulation principle: A change in the distance of a reflecting object (scanning distance) results in a shift of the corresponding light spot on the receiver and to a corresponding change in the output signals of the receiver or of the differential signal of a subsequent differential circuit. If this differential signal exceeds or falls below a difference threshold as a consequence of such a change, the evaluation unit switches to a positive or to a negative article detection signal respectively. The threshold thus corresponds to a certain switching distance.
To avoid an unwanted multiple switching between the positive and the negative article detection signal with objects which are disposed or move at the switching distance, a switching hysteresis can be provided: As soon as the sensor switches from the positive to the negative article detection signal—or vice versa—the threshold forming the basis of the further comparison of the differential signal is also changed, and indeed such that the differential signal must now carry out a sufficiently clear change in the opposite direction in order to again bring about a change in the article detection signal.
The following problem can result for some applications with the known sensor and known detection method: To ensure an unambiguous detection of objects with an irregular, reflecting surface, the switching hysteresis and/or—with respect to the scanning distance—the positioning hysteresis can admittedly be enlarged. Since, however, objects with a different reflectance can result in different differential signals with the same respective scanning distance, the positioning hysteresis set is dependent on the reflectance of the relevant object in such a case. This has the result, for example, that black objects have a larger positioning hysteresis than white objects. Such different switching distances are, however, often unwanted.